Missing MVP candidate Nikola Jokic, the Nuggets' hot run of form looked in serious jeopardy early; the Western Conference kingpins trailed by as many as 11 points and were not in front until midway through the third quarter.
Australian Giddey recorded 14 first-half points and appeared on track to outdo his personal best for points in a game (28).
Guard Jamal Murray (26 points, nine assists) led the way for the Nuggets with five assists in the third quarter as they seized the momentum.
With the game on the line late, Giddey came up with two baskets in quick succession to consolidate the Thunder's lead and force a Nuggets timeout.
The score was tied at 99-all with 11 seconds on the clock when Giddey made a no-look inbound pass to backcourt partner Shai Gilgeous-Alexander (34 points, five rebounds), who sunk an easy lay-up that ultimately won the game.
Giddey finished with 18 points and a team-high nine rebounds as the Thunder registered their 23rd win of the season.
"We've put a lot of work in," Gilgeous-Alexander said.
"When we see guys make a play at the end of the game, the close ones are the most fun ones, the emotions are a little bit higher and it feels a little bit better."
The Warriors led by 12 points with less than six minutes remaining but scored only six points for the remainder of the game as the Nets rallied.
Australian Ben Simmons was chief architect in the backcourt for the Nets with 11 assists as Curry (26 points, seven assists) and Kyrie Irving (38 points, nine assists) led the scoring for their respective teams.
The Suns were undersized after Landale and Bismack Biyombo both reached six fouls in the final five minutes of the game but edged their opponents out thanks to two Dario Saric free throws in the final 15 seconds.
New Orleans' 25th turnover came on a five-second violation with 15.1 seconds left, with the Heat up by two.
The Pelicans argued that Australian Dyson Daniels was calling for a time-out before the violation but to no avail as the Heat prevailed.
"I felt like I called time-out ... didn't get the call," Daniels said.
- with AP